1. Field of the Invention
The present invention relates to a toner for electrostatic development suitable for an image forming apparatus having a development apparatus which forms a thin layer by pressing a developing roller and a developer layer thickness control member and further having a unit which performs contact charging to the latent image bearing member, and a developer, an image forming method, an image forming apparatus and a process cartridge using the toner for electrostatic development respectively.
2. Description of the Related Art
The research and development relating to electrophotography have been conducted with all kinds of ingenuity and technical approaches. In electrophotography, a toner image is formed by developing a latent electrostatic image formed by charging and exposing the surface of a photoconductor using a color toner and an image is formed by transferring a toner image to a transferred medium such as transfer paper and by fixing it with a thermal roll.
An effective measures employed for electrophotography and electrostatic recording nowadays are dry developing methods including a method using a two-component developer made of toner and carrier and a method using a single component developer which does not contain carrier.
The two-component developer method is advantageous in handling of developers because the process for developing toner is carrier-mediated and it is easier to obtain appropriate images relatively stably when long-life and printing in high-speed region are required. However, since carrier deterioration and fluctuation of mixing ratio of toner and carrier are likely to occur, a constant quality of images in a prolonged period is difficult to obtain and there are drawbacks in maintenance and size reduction of apparatus, making single component developer method more advantageous in these points.
However, competitive regions of these methods are increasing with progress in heightening of image quality, size reduction, cost reduction, speeding up and the transition from black-and-white prints to color prints, and the scope of choices available to users is also being widened. And the operability including lightweight, compactness, exchange and maintenance, and handling are becoming important elements for office usage even with the two-component method.
Moreover, further speeding up has been extended even to the printing region and not only loading of pulverization toner but also loading of polymerization toner as a developer is increasing in both methods from the viewpoint of size reduction, conglobation and oilless fixation of toner in conjunction with these demands.
The demand for a developer with which characteristic of two-component method can be maximized, is suitable for use at a wide range of printing speeds and corresponds to simultaneous pursuit of long-life and high image quality and compact process is further increasing.
The issues associated with two-component developer to respond to above demands are as follows.
(1) Increase in charged amount associated with size reduction of the toner, electrification property change due to duration and enlargement of adhesive property
(2) charging ability change due to contamination of carrier and member
(3) adhesive condition change of external additives on the toner surface
Various proposals have been given in order to settle above issues. For example, it is disclosed in Japanese Patent Application Laid-Open (JP-A) No. 2000-98667 that the stable electrification property can be obtained by containing spicular inorganic powder and spherical inorganic powder, of which the surfaces of fine particles are coated with metal oxides in carrier coating resin, thereby preventing contamination of the carrier surface by the material used for the toner and it is stated as an indicator of contamination resistance of developer.
And in Japanese Patent (JP-B) No. 3136756, it is disclosed that the charged amount change of toner is decreased by attaching a charge control agent used for toner and a charge control agent of homopolarity on the surface of binder carrier, thereby suppressing carrier spent.
The prevention of adhesion of inorganic oxide fine particles on the photoconductor in order to prevent filming by providing resin particles nonadhesive to the toner, which has a volume average particle diameter in the range of 0.5 μm to 8.0 μm, in the developer and by applying cushioning effect to the inorganic oxide external additives between toners by the resin is disclosed in JP-A No. 2002-156782 for the developer for developing latent electrostatic images which contains toner, to which inorganic oxide fine particles are externally added, and carrier. However, this does not stabilize electrification property of the toner.
It is disclosed in JP-A No. 2003-255592 and JP-A No. 2003-255593 that the charge property of the negatively charged toner, to which hydrophobic external additives are at least externally added to the toner base particles, can be stabilized by using at least hydrophobic aluminum oxide-silica dioxide compound oxide particles and hydrophobic metal oxide fine particles, which have larger work function than that of aluminum oxide-silica dioxide compound oxide particles, as external additives. It is disclosed in the above literatures that the image quality, in which occurrence of oppositely transferred toner and fog are suppressed, negative charge property is appropriate and transfer efficiency and charge are more stable for a prolonged period, can be obtained. However, only equivalent effect compared to the toner in which silica and alumina are added separately can be obtained and when the ratio of alumina is large, there are problems of fog, electrification property and environmentally-resistant durability in particular. Being small in particle diameter is also disadvantageous in obtaining above effects and the effects on member and carrier contamination and condition change of external additives on the toner surface are also small.
The electrification charge of the toner for two-component magnetic developer in a magnetic brush developing method as disclosed in JP-B No. 2769317 can be sustained in an appropriate range during developing and transfer by regulating the permittivity and dielectric loss tangent in a predetermined range.
In JP-A No. 2004-78206, it is disclosed that the toner particles containing at least carbon black, which are used with a magnetic carrier and releasing agent, can be a black toner with excellent electrification property and transfer property by setting the loss tangent of the toner, which is expressed by dielectric loss factor ε″/permittivity ε′ at a predefined frequency, to a predetermined value.
However, the charged amount of the toner in the two-component developer method could not be uniformized sufficiently by these methods disclosed in prior art.
The single component developer method is relatively easy to employ because of its simple and compact design as a developing system which is not carrier-mediated, however, it is disadvantageous in terms of printing speed or long-life and therefore, it is useful as a compact printer in a low-speed region with which user maintenance and exchanges are easy.
In this method, a toner (developer) is transported to the developing portion normally by at least one toner feeding member and a latent electrostatic image formed on the latent image bearing member is made visible by the fed toner, however, when it is operated such a way, a layer thickness of the toner fed on the surface of the toner feeding member must be thin as much as possible. And when single component developer and a toner having a high electrical resistance are used in particular, it is required to charge the toner by a development apparatus and the layer thickness of the toner must be particularly thin. If the toner layer is thick, a portion near the surface of toner layer is only charged and it becomes unlikely for the entire toner layer to be uniformly charged.
Therefore, various methods have been proposed for control unit of toner layer thickness (developer layer thickness control member) on the toner feeding member and a representative example include a toner layer thickness control in which a control blade is positioned facing the toner feeding member and a toner layer thickness is controlled by pressing the toner fed to the surface of the toner feeding member with a pressing member (control blade). Moreover, another example may be the one in which the same effect is obtained by bringing a roller in contact instead of using the blade.
The charging method of the photoconductor surface includes a method which uses noncontact wire discharge and a method which uses contact charge injection and micro discharge and the contact charging method is preferably used for oxides suppression which occur at discharging and downsizing of apparatus.
In the toner layer formed on the surface of a developing sleeve by the developer layer thickness control member during developing, the toner is not moved from the developing sleeve to the latent image on the latent image bearing member and charge-up phenomenon is likely to occur because the toner existing near the surface of the developing sleeve in the toner layer is charged extremely high and is strongly attracted to the surface of the developing sleeve by mirroring strength, making it immobile on the surface of the developing sleeve. In particular, the charge-up phenomenon is likely to occur in low humidity.
When the charge-up phenomenon occurs, it is unlikely that the toner in the upper portion of the toner layer formed on the developing sleeve is charged, decreasing the charged amount of toner and therefore, it is likely to result in background smear, toner leakage and toner dispersal in nonimage portions.
It is necessary to have a composition in which the charged amount of the toner in the toner layer formed on the developing sleeve can be uniformly controlled as much as possible, in order to suppress the phenomenon.
Various treating agents are used to settle above issues, however, they also pose problems.
In JP-A No. 2002-31913, for example, silicate magnesium minerals (attapulgite and cepiorite) have a high moisture content and charge defects are likely to occur even in a normal usage environment and the problems caused by charge defects such as background smear, toner leakage and toner dispersal are likely to occur. Moreover, since they have low Mohs hardness, filming on the photoconductors is likely to occur, leading to image defects such as image deletion.
Moreover, when silicone oil-treated silicate magnesium as disclosed in JP-A No. 3-294864 and JP-A No. 4-214568 is used, the deterioration of toner flowability and increase in amount of charge caused by silicone oil are induced leading to transport defects and density degradation in the development apparatus.
In JP-A No. 11-95480, a two-component developer consisting of a toner coated with silicate fine powder on which the surface is treated with hexamethylsilazane in a coating ratio of 60% to 100% and a carrier on which the surface is coated with a silicone resin, having a stable electrification property and sufficient flowability is disclosed. However, when a toner with a coating ratio of 60% to 100% is prepared by using silicate magnesium as a silicate fine powder, an oppositely charged toner is likely to occur, causing background smear if it is used as a negatively charged toner. It is because silicate magnesium is likely to be positively charged due to the effect of MgO part which is likely to be strongly positively charged as shown in a relation of electronegantivity (“Journal of the Imaging Society of Japan” vol. 39 (2000) No. 3, p. 255-262).
And when a titanic acid fine powder is used as a toner disclosed in JP-A No. 11-184239, the material itself has low resistance and the charge leakage is large and likely to cause background smear, toner leakage and toner dispersal.
Moreover, since titanic acid fine powder is likely to separate from the toner, when contact charging method is employed, the contact charging member is contaminated, charge defects of the latent electrostatic image bearing member is induced leading to image defects.
When titania is used as toner as disclosed in JP-A No. 2003-186240, since this material also has low resistance and high permittivity, adjustment of the content is difficult, and when a large amount is used, the charge leakage also becomes large leading to charge deterioration of the entire toner and when a small amount is used, charged amount is increased. In either cases, background smear, toner leakage and toner dispersal are likely to occur.
And at the same time, when titania of relatively large particle diameter is used, it is likely to separate from the toner, and when a contact charging method is employed, the contact charging member is contaminated, the charge defects of the latent electrostatic image bearing member is induced leading to image defects.
Furthermore, the charged amount of the toner in single component developer method could not be sufficiently uniformized by the methods disclosed in JP-A Nos. 2002-31913, 3-294864, 4-214568, 11-95480, 11-184239, 2003-186240 and “Journal of the Imaging Society of Japan” vol. 39 (2000) No. 3, p. 255-262.